1. When Kundalini shakti reaches the Heart Center (Anahat Chakra), one attain control over lower three centers (charkas) and sees the presence of God Everywhere. This is his first glimpse of God.
If not careful, consciousness will drop to lower conters.
2. When Kundalini shakti reaches the Throat Center (Vishuddhi charka). A person now will not have any interest in worldly activities. He does not like to discuss on worldly matters
Still, If not careful, consciousness will drop to lower centers.
3. When Kundalini shakti reaches 3rd eye or center between eyebrows (Agya or ajnya charka), He realises God. He thinks his meditation is over. Still this is not the final state. There is still a thin transparent layer between him and Brahman.
Now even though a person slips his consciousness, it will not go beyond Heart Center.
4. Lastly, when the Kundalini shakti reaches the Crown Center (sahastra charka), he becomes one with Brahman. Such a person will stay in this state for 21 days and then his body drops permanently. Only avatars or some saints who are chosen by God for lok kalian (benefit of society).
Source: Sri Ramakrishna Jivan Charitra / Gospel of Sri Ramakrishna